The mortality was studied which occurred during the first 2 months of life of 9500 cross-bred chicks and 9600 Leghorn chicks. During this period 2329 chicks died in the group of cross-bred chicks and 3354 chicks died in the Leghorn group. Taking the data of both groups together, it was found that during the first week after hatching 53.3 [plus or minus] 1.06% of all chicks that died were [male][male], while the corresponding percentages were 52.1 [plus or minus] 0.59% during the 2nd to 4th week, and 53.7 [plus or minus] 0.88% from the 5th week to the end of the 2nd month. During the entire period of the first 2 months of life the [male] mortality amounted to 52.7 [plus or minus] 0.45% of the total mortality. The heavier [male] mortality appears still much more significant if the fact is taken into consideration that already at hatching time the number of [female][female] is greater than that of [male][male], the sex ratio at hatching being 48.77 [plus or minus] 0.13% [male][male]. The suggestion by some authors that the heavier [male] mortality among human infants is due to sex-linked recessive genes of a harmful nature is made improbable by these data since in fowls the [female][female] are the heterogametic sex. It is suggested that the sexual differences in metabolism account for the differential death rate of the 2 sexes.